fix (probe): skip publishing empty reports
fix (probe): skip publishing empty reports

If the probe is configured with `spy.interval` greater than
`publish.interval`, the report to be published can be completely
blank.  In that case, skip publishing it so the next time we do have
some data the time-window is correct.

weaveworks/scope,weaveworks/scope,weaveworks/scope,weaveworks/scope,weaveworks/scope,weaveworks/scope